The present invention concerns an antenna system with active modules and digital beam-forming.
In digital beam forming (DBF) signals delivered by elementary antennas, configured in an array, are used to prepare a sum signal of all these signals after they have been weighted by appropriate coefficients. When there is a phase relationship among the incident waves picked up by the elementary antennas, it can be shown that, under certain conditions, the sum signal is then the signal that would be obtained by the antenna array formed by elementary antennas, but with a relationship of illumination defined by the weighting coefficients.
DBF consists in the digital execution of this weighted summation of the signals delivered by the elementary antennas.
Moreover, an electron scanning is done by the application of a variable and controlled phase shift to the signals delivered by the elementary antennas (or, in transmission, applied to them) so that the grouping together of the different phase shifts, combined with the pitch of the array, produces a "major lobe", the direction of which, with respect to the central axis of the array, forms a variable angle, modified according to need.
Typically, a digital beam-forming antenna comprises:
a plurality of elementary antennas configured in an array;
a plurality of transmission and/or reception amplifier active modules, equal in number to the elementary antennas and each associated, respectively, with one of these modules (the term "active module" shall be taken to mean a set of active elements, such as power amplifiers for transmission, low-noise amplifiers for reception, phase-shifters etc., located in the vicinity of a radiating element of an array antenna. Generally, the energy within the active module remains at the microwave frequency of the radar).
a plurality of DBF modules, each receiving a microwave signal coming from active modules and delivering complex digital data at output, said data representing the signal received at input (the term "DBF module" shall designate such an organ, the input of which receives the microwave signal after low-noise amplification and the output of which is in the form of a complex number representing the input analog signal, namely a number with two parameters, corresponding to two channels in quadrature, called the "sine channel" and the "cosine channel"), and
DBF processing means using the complex digital data delivered by the different DBF modules of the system to prepare weighted sums of said data, wherein the weighting corresponds to a reception channel defining a narrow beam of the radiation pattern of the antenna.
At present, there are two known methods for fitting out an active array antenna with DBF modules in this way.
In the first technique, a DBF module is placed at the output of each reception channel of the active modules.
Although this approach permits every possible configuration, it has the drawback of making it necessary to have a great number of DBF modules (array antennas made at present typically comprise 4000 to 5000 elementary antennas and, hence, as many active modules).
This means paying a high penalty in the form of two consequences:
firstly, very high cost (owing to the large number of modules to be provided for),
and, secondly, a very great flow of data to be managed by the DBF processor since this processor will have to achieve real-time processing of as many complex signals as there are DBF modules, namely several thousands of complex signals.
In the second technique, the elementary antennas of the array are assembled in adjacent sub-arrays obtained by the combination of the signals coming from neighboring active modules, and only one DBF module is provided for each sub-array.
This approach, of course, very greatly diminishes the above-mentioned drawbacks since the number of DBF modules may be considerably reduced. However, it has the drawback of permitting only one good-quality beam since if there is any deviation from the direction of aim, the computed beams will have array lobes that are often unacceptable.
To aim the sub-arrays in the direction of analysis, it is necessary to provide for an electron scanning and a sequential (and no longer simultaneous as in the first example) processing of the items of information, thus making this technique costly in terms of the rate of refreshing the information when several directions have to be managed (i.e. when several beams are needed), owing to this sequential functioning mode.